The present invention relates generally to staples for bone fixation, formed of shape-memory-alloys (SMA) and other biocompatible metals and alloys. The present invention relates in particular to SMA staples of adjustable length spans.
Titanium-nickel, shape-memory alloys are biocompatible and resistant to corrosion; therefore, they are suitable for medical applications. These alloys have different phase structures, hence, different mechanical properties, at different temperatures. Information about shape memory alloys may be found, for example, on web site www.nitinol.com, by Nitinol Devices and components, copyright 1998, and in Conference information of xe2x80x9cThe Third International conference on Shape Memory and Superelastic Technologies Engineering and Biomedical Applications,xe2x80x9d held in Pacific Grove, Calif. during Apr. 30-May 4, 2000.
FIGS. 1A and 1B, together, schematically illustrate a typical temperature hysteresis, typical elastic stresses, es, in phase transitions, and typical stress-strain curves for a shape-memory alloy in the austenitic and martensitic phases. At a low temperature, the alloy is martensitic, and is soft and plastic, having a low es. At a high temperature, the alloy is austenitic and tough, having a high es. When a martensitic alloy is heated to a temperature As, the austenitic phase begins to form. Above a temperature Af, the alloy is fully austenitic. Likewise, as an austenitic alloy is cooled to a temperature Ms, the martensitic phase begins to form. Below a temperature Mf, the alloy is fully martensitic.
The temperature-dependent phase structure gives rise to shape memory. At the fully austenitic phase, under proper heat treatment and working conditions, an SMA element can be given a physical shape and xe2x80x9cpre-programmedxe2x80x9d to memorize that shape and resume it, whenever in the austenitic phase. The xe2x80x9cmemorizedxe2x80x9d SMA element may then be cooled to a martensitic phase and plastically deformed in the martensitic phase. But when heated back to the austenitic phase is will resume its memorized shape. The transformation temperature between the phases is noted as TTR.
The reason for the shape memory is found in the phase structure of the alloy. Most metals deform by atomic slip. Dislocations and atomic planes slide over one another and assume a new crystal position. In the new position, the crystal has no memory of its order prior to the deformation. With increased deformation, there is generally a work-hardening effect, in which the increased tangle of dislocations makes additional deformation more difficult. This is the case even when the increased deformation is in the direction of restoring the crystal to its original shape. However, for shape memory alloys, both transitions between the austenitic and martensitic phases and deformation in the martensitic phase change lattice angles in the crystal, uniformly for the whole crystal. The original austenitic lattice structure is xe2x80x9crememberedxe2x80x9d and can be restored.
FIG. 1C schematically illustrates typical phase structures of a shape-memory alloy, as functions of temperature and deformation, as follows:
in the austenitic phase, the crystal has a cubic structure, and the atoms in the lattice are arranged generally at right angles to each other;
when the austenitic crystal is cooled to a martensitic phase, a twinned lattice structure is formed;
when the twinned martensitic crystal is deformed by an amount no greater than xcex4, the twinned structure is xe2x80x9cstretchedxe2x80x9d so that the atoms in the lattice are arranged generally at oblique angles to each other, wherein the oblique angles are determined by the amount of deformation; and
when the deformed martensitic crystal is heated, the crystal resumes its cubic structure, wherein, again, the atoms in the lattice are arranged generally at right angles to each other.
Another property that can be imparted to SMA elements, under proper heat treatment and working conditions, is super-elasticity, or Stress-Induced Martensite (SIM). With this property, a fully austenitic SMA element, at a temperature above Af, will become martensitic and plastic under high stress, and deform under the stress. When the stress is removed, the SMA element will return to the austenitic phase and to its memorized shape in the austenitic phase. Super-elasticity is also referred to as rubber-band like property, because the SMA element behaves like a rubber band or a spring, deforming under stress and resuming its original shape when the stress is removed. However, this property is present only above the temperature Af, and only when it is specifically imparted to an SMA element, by proper heat treatment and working conditions.
FIG. 1D schematically illustrates a typical cyclic transformation of a super elastic alloy, at a constant temperature above the temperature Af. The transformation between the austenitic phase and a stress-induced martensitic phase is brought about by stress and is eliminated when the stress is removed.
It should be emphasized that both full shape memory and stress-induced superelasticity occur as long as the deformation is no greater than xcex4, and with greater deformations the crystal structure will be damaged.
Staples and clamps for bone fixation of fractures, formed of shape-memory alloys, are known. They are easily inserted in a martensitic phase, when deformed to an open, straightedge state, and they resume a closed, clamped state in the body, thus forming a closure on the fracture.
Basically, there are two approaches to working with SMA elements for bone fixation. In accordance with the first approach, the elements are fully martensitic at room temperature and are deformed and inserted into the bone when at room temperature. After insertion, the elements are locally heated to about 42-45xc2x0 C., a temperature above Af, and transform to the austenitic shape, resuming their memorized austenitic shape. The staples then cool down to body temperature, which is generally below Af, although still above Ms. Thus, in the body, the SMA elements remain austenitic and retain their austenitic shape. The advantage of this approach is that the SMA elements need not be cooled in order to remain in the martensitic phase, prior to insertion. The disadvantages, however, are that the mechanical properties of the SMA elements are not uniquely defined at body temperature, and that the SMA elements are not super-elastic in the body.
In accordance with the second approach, Af is designed below body temperature. The SMA elements are cooled to 0-5xc2x0 C., or lower, to a temperature below their Mf temperature, for deformation and insertion into the bone. Upon insertion, the elements are naturally heated to body temperature, by contact with the body only. Since body temperature is above Af, the elements transform to the austenitic phase and resume their memorized austenitic shape. The advantages of this approach are that, in the body, the SMA elements are fully austenitic, their mechanical properties are defined, and if properly heat-treated, they are super-elastic. The disadvantage, however, is that plastic deformation in the martensitic phase must be performed after the elements are cooled, and the deformed SMA elements must remain cooled during procedure manipulation and insertion.
The publication, xe2x80x9cUse of TiNiCo Shape-Memory Clamps in the Surgical Treatment of Mandibular Fractures,xe2x80x9d by Drugacz J., et al., American Association of Oral and Maxillofacial Surgeons, 0278-2391/95/5306-0006, describes a study in which clamps made of Ti50Ni48.7Co1.3, memorized to resume their shape at body temperature, were used to fix mandibular fractures. Seventy-seven patients with mandibular single or multiple fractures were treated, using 124 clamps. In 72 of the 75 patients, the treatment progressed satisfactorily, and only in five cases, infections occurred. The study concluded that the application of shape-memory clamps for surgical treatment of mandibular fractures facilitated treatment and ensured stable fixation of the bone fragments. There was no observation of pathologic tissue reaction to the clamps.
SMA staples are commercially avialable from MEMOMETAL Industries, of Cedex, France, as well as from Medical Engineering Center, Siberian Physics and Technical Institute, Tomsk, Russia, and from DePuy International Ltd., a Johnson and Johnson company, in Leeds, England, and DePuy France S.A., Cedex, France, as well as from other companies. Generally a range of shapes and sizes are offered by each company.
U.S. Pat. No. 4,665,906 to Jervis describes medical devices that incorporate stress-induced martensite alloy elments. Generally, the steps involved in the use of these devices are:
deforming a medical device into a deformed shape different from a final shape, by the formation of stress-induced martensite;
restraining the deformed shape by the application of a restraining means;
positioning the medical device and restraining means within, or in proximity to, the body;
removing the restraining means;
isothermally transforming the device from the deformed shape into the final shape.
Methods and apparatus for adjusting the length spans of bone staples are known. For example, U.S. Pat. No. 4,841,960 to Garner describes a staple whose web, or central portion, can be crimped by a pliers-like crimping device, thus shortening its length. However, this method is inappropriate for SMA elements, since the deformation will not be maintained in the austenitic shape, in the body; rather, the SMA elements will resume their memorized shape.
It is an aim of the present invention to provide apparatus and method for adjusting the length spans of SMA staples for bone fixations, prior to their insertion into the bone.
There is thus provided, in accordance with the present invention, apparatus for increasing a length span of a staple, which includes:
proximal and distal ends with respect to a user, which define a z-axis of an x;y;z coordinate system between them; and
first and second prongs, joined by a system which provides a mechanical advantage to selectably bringing said first and second prongs together and pushing them apart,
wherein said first prong further includes, at said distal end, a staple receptor, with a channel, for mounting said staple thereon, said channel defining an x-axis of the x;y;z coordinate system, parallel to said staple length span, and perpendicular to the direction of bringing first and second prongs together and pushing them apart,
wherein said second prong further includes, at said distal end, a thin, cam-like head, having a width span that increases in the direction of increasing y, operable to increase said staple length span,
and wherein, as said first and second prongs are brought together, said thin, cam-like head is arranged to slide between said staple receptor and said staple, mounted thereon, so as to wedge between said staple receptor and said staple and increase the length span of said staple.
Further in accordance with the present invention, said apparatus includes a mechanical stopping component, for controlling the amount by which said first and second prongs are brought together, hence, the length-span increase to said staple.
Additionally, in accordance with the present invention, said apparatus includes a gauge, for measuring the amount by which said first and second prongs are brought together, hence, the length-span increase to said staple.
Further in accordance with the present invention, said system which provides a mechanical advantage to selectably bringing said first and second prongs together and pushing them apart is a swivel pin.
Alternatively, said system which provides a mechanical advantage to selectably bringing said first and second prongs together and pushing them apart is a threaded bolt.
Alternatively, said system which provides a mechanical advantage to selectably bringing said first and second prongs together and pushing them apart is a pulley.
Further in accordance with the present invention, said staple is formed of an SMA alloy.
Additionally, in accordance with the present invention, said staple has an initial length span of 6 mm, wherein said apparatus is arranged for increasing said length span to a value between 6 and 10 mm.
Alternatively, said staple has an initial length span of 10 mm, wherein said apparatus is arranged for increasing said length span to a value between 10 and 14 mm.
Alternatively, said staple has an initial length span of 14 mm, wherein said apparatus is arranged for increasing said length span to a value between 14 and 18 mm.
Alternatively, said staple has an initial length span between 3 and 100 mm, wherein said apparatus is arranged for increasing said length span by an amount between 0 and 10 mm.
There is thus provided, in accordance with an alternative embodiment of the present invention, apparatus for increasing a length span of a staple, which includes:
proximal and distal ends with respect to a user; and
first and second prongs, joined by a system which provides a mechanical advantage to selectably bringing said first and second prongs together and pushing them apart,
wherein said first and second prongs further include, at said distal end, tips, arranged for mounting said staple thereon, when said prongs are brought together,
and wherein, as said first and second prongs are pushed apart, said tips pry said staple, mounted thereon, wider, thus increasing the length span of said staple.
There is thus also provided, in accordance with the present invention, a method of increasing a length span of a staple, which includes the steps of:
employing prongs which define a z-axis of an x;y;z coordinate system, generally parallel with their longitudinal axis;
mounting the staple on a staple receptor, which is arranged on the first prong, and which defines an x-axis of the x;y;z coordinate system, parallel with a length direction of the staple; and
sliding a thin cam, arranged on a second prong, and having a width which increases in the direction of increasing y, between the staple receptor and the staple mounted thereon, thus wedging the thin cam between the staple receptor and the staple; and
plastically deforming the staple, to increase its length span.
Further in accordance with the present invention, said step of sliding a thin cam further includes sliding by a predetermined amount, thus predetermining the length-span increase of the staple.
Additionally, in accordance with the present invention, the staple is formed of a shape-memory alloy having a fully martensitic phase within a first temperature range, and having a fully austenitic phase within a second temperature range, which is higher than the first temperature range, wherein said step of plastically deforming the staple includes plastically deforming the staple by reversible martensitic deformation.
Further in accordance with the present invention, said step of plastically deforming the staple by reversible martensitic deformation includes plastically deforming the staple at a temperature range of the fully martensitic phase.
Alternatively, said step of plastically deforming the staple by reversible martensitic deformation includes plastically deforming the staple in a stress-induced martensitic phase at a temperature range of the fully austenitic phase.
There is thus also provided, in accordance with the present invention, a method of bone fixation with an SMA staple, which includes the steps of:
drilling at least one pair of bores across a fracture interface of a bone;
measuring the distance span between the two bores of the bore pair;
selecting an SMA staple having a length span which is smaller than the distance span;
plastically deforming the staple, to increase its length span;
inserting the staple into the bores; and
employing the staple in the plastically deformed state, which resulted from the length-span increase.
There is thus also provided, in accordance with the present invention, a method of increasing a length span of a staple, which includes the steps of:
mounting the staple on two tips that are arranged for receiving the staple when they are brought together; and
plastically deforming the staple by prying the tips apart, to increase the length span of the staple.
Additionally, said step of plastically deforming the staple by prying the tips apart further includes prying by a predetermined amount.
There is thus also provided, in accordance with the present invention, a staple for bone fixation, formed of a shape-memory alloy having a fully martensitic phase within a first temperature range, and having a fully austenitic phase within a second temperature range, which is higher than the first temperature range, which includes:
a web having a first length span and a thickness;
two bending points, forming the end points of said web; and
two semicircular end sections, beginning from said bending points, having a radius of curvature, an angle of curvature that is greater than 90xc2x0, and a thickness which is substantially the same as said web thickness,
wherein by plastically deforming said staple, reversibly, in the fully martensitic phase, to decrease said angle of curvature to 90xc2x0, said semicircular end sections are straightened, to facilitate insertion into the bone, and said length span may be increased to a desired value,
and wherein upon transformation to its austenitic shape, said staple generally resumes its original shape, but with a second length span that is greater than said first length span.
There is thus also provided, in accordance with the present invention, a method of bone fixation, which includes the steps of:
drilling at least one pair of bores across a fracture interface of a bone;
measuring the distance span between the two bores of the bore pair;
employing a staple for bone fixation, formed of a shape-memory alloy having a fully martensitic phase within a first temperature range, and having a rally austenitic phase within a second temperature range, which is higher than the first temperature range, which includes:
a web having a length span; and
two semicircular end sections, having angles of curvature that are greater than 90xc2x0;
plastically deforming the staple, reversibly, in its martensitic phase, to simultaneously decrease said angle of curvature to 90xc2x0, thus straightening the semicircular end sections, to facilitate insertion into the bone, and to increase the length span of the web to a desired value;
inserting the staple into the bores; and
employing the staple in the plastically deformed state, which resulted from the length-span increase.
Additionally, in accordance with the present invention, said step of plastically deforming the staple, reversibly, in its martensitic phase, includes plastically deforming the staple at a temperature range of the fully martensitic phase.
Alternatively, said step of plastically deforming the staple, reversibly, in its martensitic phase, includes plastically deforming the staple in a stress-induced martensitic phase at a temperature range of the fully austenitic phase.
Further in accordance with the present invention, said method further includes plastically deforming the staple to increase the length span to a value which is substantially the same value as the distance span between the two bores of the bore pair.
Additionally, in accordance with the present invention, said step of plastically deforming includes plastically deforming to a strain that is less than 15%.
There is thus also provided, in accordance with the present invention, a method of bone fixation, which includes the steps of:
drilling at least one pair of bores across a fracture interface of a bone;
measuring the distance span between the two bores of the bore pair;
employing a staple for bone fixation, formed of a shape-memory alloy having a fully martensitic phase within a first temperature range, and having a fully austenitic phase within a second temperature range, which is higher than the first temperature range, which includes:
a web having a length span; and
two semicircular end sections, having angles of curvature that are greater than 90xc2x0;
plastically deforming the staple, reversibly, in its martensitic phase, to simultaneously decrease said angle of curvature to 90xc2x0, thus straightening the semicircular end sections, to facilitate insertion into the bone, and to increase the length span of the web to a desired value;
inserting the staple into the bores; and
employing the staple in a partially plastically deformed state, resulting from the length-span increase.
There is thus also provided, in accordance with the present invention, a staple for bone fixation which includes:
a web having:
a length span;
a curvature; and
a thickness,
wherein said staple may be plastically deformed by straightening its curvature, to increase its length span, and wherein the staple is employed in its plastically deformed state.
Additionally, said web includes more than one curvature.